It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K.
Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) and in the system Y-Ba-Cu-O (Wu et al, Phys. Rev. Lett. 58, 908-910 [1987]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and Tl has been reported (papers in press). In preparing the system Tl-Ba-Cu-O, Z. Z. Sheng and A. M. Hermann (Superconductivity in the Rare Earth-Free Tl-Ba-Cu-O System above Liquid Nitrogen Temperature) (communication from the authors), first mixed and ground BaCO.sub.3 and CuO to obtain a product which they heated, then intermittently reground to obtain a uniform black Ba-Cu-Oxide powder, which was then mixed with Tl.sub.2 O.sub.3, ground, and heated, with formation of a superconducting material. It was noted that the Tl oxide partially melted and partially vaporized.
The superconductor system Tl-Ca-Ba-Cu-O was also reported in a paper by Sheng and Hermann, "Bulk Superconductivity at 120K in the Tl-Ca-Ba-Cu-O System" (communication from the authors). The authors reported "stable and reproducible bulk superconductivity above 120K with zero resistance above 100K". According to the paper the composition was prepared by mixing and grinding together Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4. The ground mixture was pressed into a pellet and heated in flowing oxygen. The result was cooled and found to be superconducting.
Our invention is an improvement in the latter Sheng-Hermann process of making Tl-Ca-Ba-Cu-O superconductors.
See also the paper by Hazen et al, "100K Superconducting Phases in the Tl-Ca-Ba-Cu-O System" (communication from the authors) which refers to two superconducting phases, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+ .delta. and Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8+ , both with onset T.sub.c near 120K and zero resistivity at 100K. Preparation included grinding together Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 (or Ba.sub.2 Cu.sub.3 O.sub.5), followed by heating.
And see "Nota Bene" in High T.sub.c Update, vol. 2, No. 6, p. 1, Mar. 15, 1988, further re properties of the Tl-Ca-Ba-Cu-O system.